1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor carrying a radiation image stored therein to stimulating rays which cause the stimulable phosphor to emit light in proportion to the radiation energy stored, guiding the emitted light to a photodetector via a light guide member, and photoelectrically detecting the radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by th phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed, for example, in U.S. Pat. No. 4,239,968, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. In the radiation image recording and reproducing system, the energy of the light emitted by the stimulable phosphor when it is exposed to stimulating rays is much smaller than the energy of stimulating rays (approximately 10.sup.-5 times the energy of stimulating rays), and the emitted light is non-directional. Therefore, when the read-out apparatus is fabricated, the light receiving solid angle of the light guide member should be made as large as possible to guide as much emitted light as possible and improve the S/N ratio.
Accordingly, the Applicant proposed in U.S. Pat. No. 4,302,671 to provide the light receiving face of the photodetector, which is positioned close to a stimulable phosphor sheet in face-to-face relation, with a reflecting mirror or a triangular prism sufficiently smaller than the area of the light receiving face of the photodetector so that the reflecting mirror or the triangular prism protrudes from the light receiving face. In this technique, stimulating rays such as a laser beam incident laterally and exhibiting high directivity are reflected by the reflecting mirror or the triangular prism onto the stimulable phosphor sheet. However, in this case, a space at least equal to the amount of protrusion of the reflecting mirror or the triangular prism must be left between the light receiving face and the stimulable phosphor sheet. Therefore, the light receiving solid angle of the light guide member becomes small.
The Applicant also proposed in U.S. Pat. No. 4,346,295 to position a light guide member having a special shape so that one end face (light input face) is close to a stimulable phosphor sheet in face-to-face relation thereto along a scanning line of stimulating rays, and to guide light emitted by the stimulable phosphor sheet upon stimulation thereof to a photodetector by use of the light guide member. In order to efficiently guide light emitted by the stimulable phosphor sheet upon stimulation thereof and entering from the light input face by total reflection inside of the light guide member up to a light output face, the light guide member is fabricated so that the light input face is shaped in a linear form and the light output face is formed in a ring shape by gradually bending the light guide member from the light input face to the light output face. Since the light guide member is fabricated in such a shape, the size thereof inevitably becomes large, and is an obstacle to reduction of the size of the overall radiation image read-out apparatus.